This is a common ailment called Powdery Mildew. It happens during periods of moist, humid, or dewy weather. It really won't hurt your plant and at this late time of year, it's not worth attacking since the leaves will fall on their own soon anyway. Lilacs get Powdery Mildew a lot so if your plants get this every year, begin spraying them with a general purpose fungicide in the early summer after the flowers fade. Follow label directions."